Apparatus for producing pressure pulses in boreholes



Oct`- 19, 1948. A. BLANCHARD 2,451,797

APPARATUS FOR PRODUCING PRESSURE PULSES IN BORE HOLES 0ct. 19, 1948. A. BLANCHARD 2,451,797

APPARATUS FOR PRODUCING PRESSURE PULSES IN BORE HOLES. Filed Feb. 17, 1945 4 Sheets-Sheet 2 Oct. 19,4 1948. A. BLANCHARD 2,451,797

APPARATUS FOR PRODUCING PRESSURE PULSES IN BORE HOLES.

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N\\ \\\%\N% Erm wm mm. rw@ H\ wS. CN n mm W/ .v ril.: WA NQ M/ m if NQ\ i kmr N @Ww w .mwN m W m mm A x Inasmuch as the Patented Oct. 19, r1948 amr APPARATUS FOR PRODUCING PRESSUR PULSES IN BOREHOLES Andr Blanchard, Houston, Tex., assignor to Surveying Corporation, Houston, Tex., a corporation of Delaware Application February 17, 1945, Serial No. 578,529

Schlumberger Well This invention relates to devices for producing vibrations and pressure pulses, and it relates particularly to devices for emitting vibrations and pressure pulses while subjected .to high static pressures such as are encountered in fluid-lled I bore holes.

As disclosed in the Doll application, Serial No. 446,888, filed June 13, 1942, now Patent No. 2,433,746, dated December 30, 1947, it has been discovered that permeable strata in the earth which are traversed by -a bore 'hole can be located and classified by detecting alternating streaming potentials produced in such strata. The alternating streaming potentials can be produced lby generating ypressure pulses in the bore hole liquid by means of pressure pulse emitting devices similar to those used in submarine signalling. v

The prior submarine signalling devices are useful at shallow depths wherein the static pressure of the liquid does not exceed a few hundred pounds per square inch. However, these devices are not satisfactory for use atgreater depths wherein the static liquid pressure may be several thousand pounds per square inch.

Such prior devices are usually provided with a diaphragm in contact made expansible and contractible in order to gencrate the pressure pulses. In order to withstand the very high pressure encountered in the bore hole, i-t ls necessary to make the diaphragm and the casing of the device very thick 'and rigid, and 'for that reason it lis necessary to provide a very powerful mechanism for vibrating 4the diaphragm. bore hole, at great depths, for example 8,000 to 10,000 feet, usually has a diameter of only about seven or eight inches, it is difiicult to provide a vibrating mechanism' of suicient power to generate substantial pressure pulses. Moreover, the stiffness of the diaphragm tends to increase Iits natural resonant frequency and inasmuch as the generator must be operated at its natural frequency in order to be reasonably efficient, such generators vare operable only at very high frequencies.

An object of the present invention is to provide anl apparatus which is capable of producing powerful pressure pulses evenvwhen subjected to very high static pressures.

Another object of the invention is to'provide an apparatus capable of producing powerful pressure pulses and which at the same time is suiciently compact to permit it to be lowered into a bore hole.

Another object of the invention is to provide with the liquid or arev 12 Claims. (Cl. 177-386) 2 a signal generator lwhich does not include a. diaphragm and which does not appre-ciably change its volume while in use.-

A further object .of the invention is to provide electro-mechanical oscillating systems which are adapted to operate at a predetermined frequency or at a self-maintained natural resonant frequency.

Other objects of the invention will become apparent from the following description of typical forms of signal generators embodying the present invention.

In accordance with .the present invention av signal generator has been provided including an element, hereinafter referred to as ra bell, which is vibrated bodily, without change of volume, under such conditions as to produce powerful pressure pulses in the bore hole fluid.

More particularly, Vdevices of the type embodying the present invention may include a relatively massive body containing electrical equipment necessary for supplying energy to' a vibrating mechanism in lthe bell. thereto by a resilient coupling so that upon operation of a vibrating mechanism in the bell, it is caused to vibrate bodily relatively to the more massive casing.

In one form of the invention, the bell may be vibrated at its natural resonant frequency by means of an oscillator tuned to this frequency so that under all conditions of operation, the frequency of the vibrating system remains the same.

In another form of the invention, a bell may be vibrated at its natural resonant frequency and at a self-maintained frequency whereby the frequency of vibration of the bell may vary in accordance with the density of the liquid in the bore hole and other factors. y

Inasmuch as the bell is not expansible or contractible, it can be provided with strong rigid walls which permit it to be used at substantially any depth regard-less of the pressure exerted on it by the fluid.

Devices of this character are particularly suitable for generating alternating streaming potentials in porous strata at all depths in the bore hole', inasmuch as the device is operative regardless of the depth at which it is used. This is a great advantage inasmuch as the measurements of the potenti-als are usually made while moving vthe transmitter or emitter along the bore hole.

For a better understanding of the present invention, reference may be had to the accompanying drawing in which:

The ybell is connected Figure l isa view in vertical section of a typical form of signal generator operable at a xed frequency, embodying the present invention:

Figure 1A is a view taken on line iA-IA of Figure 1 looking in the direction of the arrows;

Figure 2 is a view in vertical section of a modified generator which operates at its resonant frequency; y

Figure 2A is a detail of the pickup device of Figure 2;

Figure 3 is a typical wiring diagram which may be employed with the apparatus of Figure 1:

Figure 4 is a typical wiring diagram which may be employed with the apparatus of Figure 2;

Figure 5 is a view in vertical section through a modiiled type of generator which incorporates a tuning fork:

Figure 6 is a perspective view of the tuning fork and bell assembly of Figure 5;

Figure 7 is a plan view of a coil mounting -member which is rigid in two directions but very exible in the third direction;

Figure 8 is a vertical section' through a modified form of signal generator; and

Figure 9 is a vertical section through another type of signal generator.

A'feature of this invention resides in the provision of a signal generator, which generator is vibrated bodily at the desired frequency. The driving device may be of any desired type. It is preferably an electrically energized mechanical arrangement having a pair or more of masses connected by an elastic member. Typical examples oi' this class of generators will be described in detail to disclose the novel principles of the invention.

Referring to Figure 1, the apparatus is shown positioned in a high pressure media such as the fluid F contained-in a deep bore hole H. An

upper, water-tight, pressure resistant casing i of L relatively large mass houses an electronic amplier and detector 2 and an electronic oscillator I of conventional design. 'Ihe lower end of casing i has a protruding hollow threaded pin l onto which is screwed the top end of a metal tube 8 which is suillciently elastic to stretch and contract under axially directed stresses. Pin l and tube B receive two insulated wires 1 and l. The lower end 9 of tube 6 is enlarged and threaded internally to permit it to be connected rigidly to a pressure-resistant, water-tight casing or bell Il.

The bell I0 is a hollow, liquid-tight casing. preferably, although not necessarily, cylindrical in shape.

It is desirable sometimes to have more than one elastic member 8 between the bell I0 and upper housing i, in which case, the tube 8 may be replaced by equivalent means such as three equally spaced rods. A

The length of the bell i0 preferably should be several times its diameter, the best length being half a wave length in the bore hole fluid.

The bell i0 contains a mechanical oscillator system, hereinafter described.

The entire outer surface of the bell i0 and part or all of the tube 6 are covered with an insulating material Il. Two electrodes i2 and i3 are positioned near the top and bottom ends of the bell I0 respectively, and the dimensions oi' the entire apparatus are chosen so that the electrodes I2 and Il will be in close proximity to the walls of the bore hole H. Electrodes i2 and i2 are connected by shielded insulated wires il and ila respectively to the input of the amplifier and detector 2.

The apparatus is lowered into the bore hole on an electric cable containing insulated electrical conductors Il, Il, i1, and Il.

The mechanical oscillating system includes an electromagnet i! having a laminated magnetic core 2li within which is formed a winding 2|, the terminals of the winding 2i being connected by means of the conductors l and l tothe output of oscillator 2. .A high permeability laminated armaturen is so arranged that it completes the magnetic circuit ofthe electromagnet Il in a conventional manner. The magnetic core 2l is rigidly fastened to the base of bell il by a clamping plate 23.

The, armature 22 is rigidly attached to a meinber 24, the upper end of which is rigidly attached to the top end of the bell Il and the tube I. The member 2l is resilient so that its overall length changes as the armature 22 is attracted to the magnetic core. In order to make the member 24 more resilient, a frame-like member 25 is formed intermediate its two ends. The member 2l is essentially a spring, the stiffness of which il altered by increasing or decreasing its dimensions. The length of elastic member 24 is such that a small air gap normally exists between the armature 22 and the core 20.

In operation the apparatus is lowered into the bore hole and the electrical energy for producing the vibrations is supplied from the surface through the conductors i5 and il in the supporting cable, as better shown in Figure 3, or from a battery (not shown) in the casing I. The electrical energy may be either alternating or direct current as desired, but it will be assumed that 60 cycle alternating current is supplied. The 60 cycle energy sent from the surface supplies the nlament current. plate voltage, etc. for both the amplifier and detector 2 and the oscillator 2 in the conventional manner. The oscillator 2 is adjusted` to a frequency which corresponds to the natural frequency of the mechanical oscillating system 22, 2l and the bell l0 as will be more fully explained later. The output of oscillator 3 is transmitted through wires 1 and l to the electromagnet I9, thereby energizing the electromagnet at its natural frequency. When current flows through the winding 2| of the electromagnet, the ux produced passes out of the core 2li through the air gaps 26 and 21, the magnetic circuit being completed by the armature 22. The output of oscillator 3 is biased by a constant voltage so that its output takes the form of a modulated wave, that is, the current always ilows in the same direction through the winding 2i, but the magnitude varies at the oscillator frequency. Thus the action of the magnetic field produced by electromagnet I8 is topull the armature 22 toward the core 20 once each cycle.

Alternately, the oscillator frequency may be half the natural frequency of the mechanical oscillating system and the electromagnet is energized by the unbiased output of the oscillator. The electromagnet then pulls downwardly on armature 22 twice each cycle, driving the mechanical oscillating system at its natural frequency. The frequency of vibration may be of any desired value, for example, 600 cycles per second,

The upper case i, having a relatively very large mass, does not follow appreciably the vertical or axial vibrationsV of the bell l0. The tube I, therefore, is alternately compressed and elongated at the vibration frequency so that the bell il has a vertical motion of small amplitude.

This movement develops an inertia force which is compensated for bythe elastic force of the tube, thus making the whole system mechanically resonant.

If at one instant of time the bell I is moving downwardly, the pressure below the bell is'increased and the pressure above the bell is decreased. An alternating differential pressure, therefore, exists in the bore hole fluid above and below the bell I0. The bore hole pressure at a point opposite electrode I2, therefore, alternately increases and decreases. The same pressure pulses, but of opposite phase, appear in the bore hole adjacent theelectrode I3. These pressure pulses produce alternating streaming potentials in the adjacent permeable formations, which potentials are picked up by the electrodes I2 and I8 in a manner and for purposes more fully explained in co-pendingDoll application, Serial No. 446,888. The potential differences appearing between electrodes I2 and I3 are conducted through the well shielded wires It and Ida to the input of the amplifier and detector 2. These small voltages are amplified and then rectified by unit 2 and the D. C. output or any signal controlled by it is transmitted through the two conductors I1 and I8 in the supporting cable to the surface of the earth where they are recorded by suitable recording means.

In a mechanical resonant oscillator like the device described above, the amplitudes are limited by the friction and losses in the mechanical system. For efficient operation of a mechanical oscillator, it is desirable that no rubbing or undue friction loss be present and,` consequently, that every part of the vibrating system be very rigidly held together. For these reasons, it is necessary to take great precautions to make sure that the entire vibrating system is very rigid so that it may function as one mass.

One diiiculty encountered in the construction of this type of apparatus is in providing a coil winding which will function as a solid mass. This dimculty can be overcome by supporting the coil in such a manner that it need not follow the vertical vibrations of the bell I0. Figure 1A discloses one means for supporting the winding of electromagnet I9 to permit relative movement of the bell I0 and the winding.

Referring to Figure 1A, the 'winding- 2| extends beyond the core 20 Aon two sides. Soft compression springs 29 are mounted between the winding 2l and the base of the bell Ill. The stiffness of the springs 29 and the mass of the winding 2| are so arranged that the natural resonant frequency is very-low, for example, on the order of 2O to 30 cycles-per second so that for all practical purposes the winding 2i remains stationary while the core 20 and bell I0 vibrate vertically. Also with this construction, the mass, center of gravity, etc., of the winding need not be considered when computing or determining the resonant frequency of the mechanical oscillating system.

When two electrical circuits are to be coupled,

the degree of coupling between the two circuits must not be too large or to small, if a single resonant frequency of maximum amplitude is to appear in the output.` When the two coupled circuits are loosely coupled, part of the amplitude is lost. When the two electrical circuits are overcoupled, the Well known double humped frequency characteristic appears; that is, the combined circuit actually has two resonant frequencies.

mum movement of the vibrating system, it isl necessary that the natural frequency .of the bell be approximately thesame as the natural frequency of the armature and its support, and the degree of coupling between the twoA should be` proper. This coupling 'mayv be altered by changing the physical dimensions of the spring '25, for p example, if less coupling is desired, the thickness of the metal in spring 25 is decreased and the mass of armature 22 is decreased so that the natural frequency is maintained. The proper coupling is usually determined experimentally.

As the potentials appearing at the electrodes I2 and I3, and hence inthe conductors I6 and 14a, are very small compared'to the energy in the oscillator and electromagnet circuit, the two circuits should be well shielded one from the other and, if convenient.' each pair of wires should be twisted together. The same is true, but to a lesser degree, of the wires in the supporting cablc.

The bore hole signals derived from electrodes I2 and I3 are amplified and changed to a variable direct current by the amplifier and detector 2 so that the signal level in the conductors I1 If the amplifier and the oscillator are not mounted in ferromagnetic cases, a good electromagnetic shield 30 should be provided between the two units. v

A modified form of signal emitting device which operates at a self-maintained frequency isdisclosed in Figures 2 and 4.

Referring to Figure 2, most of the parts, construction, and function of the devices disclosed therein are identical wtih those described in Figure l, and'corresponding primed numerals have been applied where applicable. A

.A pickup device 40 similar to an electric phonograph pickup is attachedto vbell I0 in any suitable fashion. The pickup device 4|)v may be of any conventional design, and, as shown, it includes a permanent magnet 4I, an armature 42 mounted in. the magnetic eld and a coil 42a around the armature. In a conventional pickup devicethe permanent magnet is held stationary and the voltages are induced as a result of. the motion of the armature. 'I'he same may be followed in this case, but it has been found more practical to attach the permanent magnet Il to the bell and to control the movement of the armature by inertia. 'I'his may be done by fixing a short arm 43 and a small weight 45 to the elastic system on which the armature. is already mounted as shown in greater detail in Figure 2A. The armature is pivoted as at 45 andthe armature is urged to its middle position by a spring 46. The mass of the weight 44 depends Vthen shifting the phase 180 but even then mass 44 is small, for example only a fraction of ran ounce. While this small system comprising armature 42, which pivots about point 45, the arm 43 the weight 44, and the `spring 46 is not rigidly fixed, it has a natural frequency much lower than the vibrating frequency of the bell so that the weight 44 will not follow the motion of the bell. For example, the resonant frequency of the armature 42. arm 43, spring 46 and weight 44 may be 20 or 30 cycles per second as compared to a vibrating frequency of 600 cycles per second.

In this form of the invention, the oscillator 3 in Figure 1 is replaced by a conventional AVC amplifier 41. The electrical circuit is shown in Figure 4. Referring to Figure 4, the output of the pickup 40 is fed to the input of a wide band amplifier 4l, the output of which drives the electromagnet I9'. The remainder of the circuit of Figure 4 is the same as that of Figure 3 and need not l be described in detail.

The operation of the modification of Figures 2 and 4 is practically identical to that described A`in connection with Figures 1 and 3. The apparatus is lowered into the bore hole on the electric cable, and due to spurious vibrations, the armature 22' is set in motion at its resonant frequency. As soon as the armature starts to vibrate, the vertical motion of the ybell I0' sets up a corresponding voltage inthe pickup unit 40. The voltage derived from pickup 40 is fed into the amplifier 41, the output of which drives the electromagnet I9', as stated before, so thata regenerative system is provided. The natural resonant frequency of the armature 22 and its elastic support 24 remains constant for allA practical purposes, but the resonant frequency of the bell changes with the density of the bore hole iluid and other factors, so there are now two coupled mechanicaloscillating systems and the frequency of the two systems need not be the same. The above mentioned double-bumped frequency characteristics may now appear in the overall system. The mechanical coupling may be altered so that the two oscillating systems are overcoupled; one of the two resonant frequencies is more pronounced than the other. The overall resonant system may, therefore, be maintained on the hump having the largest magnitude; for example, the system may be maintained on the lower resonant frequency.

When the double hump frequency characteristie is present, the phase of one hump is opposite to the phase of the other hump, that is, in terms of 'phase alone, the'two humps are 180 apart. By controlling the phase between the pickup device 40 and the amplier 41, the resonant system will be maintained on the frequency hump selected. It may be advisable in some cases to include a conventional phase shift network 54 containing resistances and condensers between the pickup 40 and the input of amplifier 41. If the phase is adjusted to maintain the system on one frequency hump, for instance the lower frequency hump, causes the system to resonate continuously on the higher frequency hump.

Accordingly, the frequency of the bell i0 will change automatically with changes n the fluid density or other factors, and a log of frequency versus depth may be made if desired. Such a log is particularly useful in studying the bore hole fluid, as ows of water, gas, or oil into the bore Another arrangement for vibrating the belll vertically is shown in Figures 5. 6, and 7. The apparatus of Figures 5. 6, and 'I may be substituted-for the bell I. and its contents in Figures 1 or 2; that is,-the modification 'shown in Figures 5, 6, and 7 isadaptable to a signal generator of either fixed or variable frequency.

If a constant frequency device is desired, the schematic circuit diagram of Figure 3 will be employed, or if a variable frequency acoustic generator is desired the principles shown in the schemati-c circuit diagram of Figure 4 will be used.

Figure 5 is va vertical section, and Figure 8 is a perspective view of an acoustic generator wherein a tuning forkA is employed to vibrate the bell. The bell 50 contains a tuning fork Ii which is adapted to 'be continuously energized by an electromagnet 52. The electrical energy for driving the tuning fork is supplied, as described above, to the winding 53 of the electromagnet 52 which drives the tuning fork 5i at its natural frequency. When current is flowing through the winding N of the electromagnet. the magneticflux produced by the core passes out of one ofy the pole pieces l1 through the air gaps, through the tines l5 and 'Il of the tuning fork, to the other pole piece l1, completing the magnetic circuit. "Ihus, the action of the magneti-c eld is to pull both tines of the fork toward the electromagnet each cycle-or each half cycle as the case may be, so the tuning fork is continuously energized at its natural frequency. The tuning fork and the pole pieces of the electromagnet should be designed so the air gaps are very small yet such that the tuning fork never actually touches either of the pole pieces. Longitudinal grooves formed part way through the tines of the tuning forkreduce the eddy currents.

When the tines of the tuning fork vibrate in opposition, a vertical component of force is present in the stem of the tuning fork, imparting a vertical or axial motion, only, to the bell l0.

Inasmuch as it is necessary to take great precautions to make sure that the entire vibrating system is very rigid so that it may function as one mass, the entire electromagnet may be mounted so that neither the laminated core nor the winding follows the vertical motion of the bell and hence the electromagnet 52 'cannot absorb appreciable power.

Referring to Figure 5, the electromagnet 52 is held in place by means of two special mountings, the details of which are better shown in Figure 7. A thin flat sheet 59 of metal.' such as .steelf is formed having two or more slots 60 and 6| which are substantially the length of the sheet. The sheet 59 is fastened to the bell by means of screws or bolts which pass through holes 62 and 63 in the outer portion of the sheet. The holes 62 and 83 are preferably located on the transverse axis of the sheet as shown in the drawing. The electromagnet 52 is attached to the sheet 59 by a screw which passes through a hole 64 in the center of the sheet. A sheet 5l is provided for both the top and bottom of the electromagnet. Referring to Figure 7. it is evident that the center of each sheet may move very freely in the direction perpendicular to its plane but that the sheet is very rigid in any direction parallel to its plane. Thus the electromagnet 52 is held very rigidly in a horizontal direction so that it may be well centered between the tines of the tuning fork, but it may move freely in a vertical direction. As the electromagnet 62 has considerable mass, and as the bell 50 moves in. a Avertical direction only, the electromagnet tends to remain 4stationary and does not follow'the vibrations, and hence it does not interfere with the vibration of the bell.

The sheet 59 may befastened directly to the bell by suitable fasteners passing through holes 62 and 68. However, it is diillcult to assemble and adjust the unit so the rod construction shown in Figure 6 is preferred. Any suitable number of stiill rods 65 are rigidly attached to the top portion 50' of the bell 50. The sheets 59, 59 are then fastened to the rods 65, as shown, and the electromagnet mounted on the sheets as described. The tuning fork is also mounted on the removable top end 50of bell. Thus all the parts are accessible and may be easily adjusted. When completely adjusted, the bottom 50" of bell 50 is passed over the assembly and parts 50 and 50" are screwed together. The insulation is then applied over the completed assembly.

'I'he tuning fork and bell assembly shown in Figures and 6 constitute two coupled mechanical oscillating systems and each of these mechanical oscillators may have its own frequency. Therefore, in order to obtain the maximum power output from the vibrating system, it is necessary Each of the two masses 8| and 82 is supported from and fastened to the base of bell 80 by means 82 provide space for a coil winding 81 which is disposed on an insulating spool 88. The spool that the natural frequency of the bell 50 be approximately the same as the natural frequency of the tuning fork, and the degree of coupling between the two should be proper. The coupling` may be altered by changing the curvature and the physical dimensions of the throat 86 of the tuning fork.

In order that the system have frequency sta` stem of the tuning fork orto the beu 5u and' byusing the principles described in connection with the devices disclosed in Figures 2 and 4, the frequency may'vary as ambient conditions change.

Other arrangements for vibrating the bell vertically are illustrated in Figures 8 and 9. These modifications have the advantage that the constants are essentially lumped and consequently the design and computations are much simpler than for a tuning fork, for the mass of the tines of the tuning fork is distributed throughout their length and their center of gravity is not well defined. Also the axis of rotation of the moving masses is well defined in the modifications of Figures 9 and 10 so that it is easy to compute the natural frequency of such a system. The frequencymay be easily changed as desired. Also in the arrangements shown, the mechanical coupling between the two mechanical oscillating systems may be easily adjusted in either direction.

Referring to Figure 8, the bell 80 may be the same as was discussed in connection with Figures 1 and 5, and the apparatus above the bell is identical to that previously explained, that is, the bell is supported from an elastic rod which is in turn fastened to a relatively large mass containing an oscillator, amplifier, and so forth. The feature of the modification illustrated in Figure 8 is the means for vibrating the bell.

Two laminated masses 8| and'82 are connected together by means of a U-shaped spring 83.

88 and its winding 81 are supported by four or more arms 89 which are fastened to the side walls of the bell 80.

1n operation the winding 81 is energized from a suitable source such as an oscillator or ampli- ,ner 8.l When the winding 81 is energized. the resultant magnetic field urges the two masses 8| and 82 toward one another. This motion is resisted by spring 83 so that when the winding 81 is deenergized, the spring 88 urges these two masses 8| and 82 away from one another. When the two masses 8| and 82 move toward or away from one another. their paths essentially follow arcs the centers of which are the axes of rotation S0 and 9| respectively in much the same fashion as the apparent mass of one time ofthe tuning fork moves about a point in the yoke of the fork. The motion of the two masses 8| and 82 about their axes of rotation and 9|,- respectively, is resolved into horizontal and vertical components, with the result that the two horizontal components are always equal and opposite and cancel while the two vertical components are always in phase and additive to provide axial vibration of the bell 80. While the resultant vibrations in a vertical direction are the same as those of Figure 5, the critical adjustments may be much more easily varied in the modification of Figure 8. For example, if a higher resonant frequency is desired, the two legs of the spring 83 may be made thicker or they may be shorter; thus the resonant frequency may be conveniently altered Without changing any of the other characteristics of the acoustic generator. The coupling between the two mechanical oscillating systems, that is, the coupling between masses 8| and 82 and the bell 80, may be conveniently altered by moving the pivot members 86 and 85 closer together or farther apart as desired. The coupling may thus be adjusted in either direction as often as convement.

In order to construct a coil which does not affect the vibrating masses, the spool88 and the winding 81 are suspended from the resilient memy l bers 89 so that the natural frequency of the coil and members 89 is very low. In this Way the coil 81 does not follow the vertical` motion of the bell. The members 89 are similar in construction and function to the sheets 59 in Figures 5 and 6.

Another construction which operates upon the bers and 96 are suspended on members 91 and` 98 which function as pivots. The elastic member 99 between the two vibrating members`95 and 96 takes the -form of a tore or a flat ring to which each of the vibrating members 95 or 96 is rigidly attached.

A coil winding |00 is formed on a magnetic core |0| having pole pieces |02 and |03. Two high permeability paths for the magnetic flux are, therefore, created by core 0|, pole piece |02, air gaps |04 and |04', members 95 and 96, the airgaps |05and |05', and pole piece 03. The entire assembly including parts |00, |0|, |02, and |08 is supported at each end `by resilient plates |06 which are similar to the sheets 59 shown in l l Figure '7; thus the assembly may move freely in a vertical direction but is rigidly restrained in all lateral directions, maintaining the air gap uniform. The operation is practically identical to that previously described in connection with Figure 8. The winding is energized from a suitable source such as an amplifier or oscillator which creates a magnetic ilux the strength of which varies at the same frequency, or at double the oscillator frequency, as desired. The vibrating members 95 and 99 are, therefore, pulled to ward one another when the coil |00 is energized. When the coil |00 is deenergized. the members 95 and 96 are urged apart by the .action of the circular spring 99. As the two masses 95 and 99 described arcs about their centers of rotation |01 and |09. respectively, the circular spring 99 is alternately compressed and expanded so that it beccmesan ellipse, the long axis of which coincides with the plane of members 95 and 96, then a circle in its neutral position, then an ellipse with its short axis coinciding with the planes of vibrating members 95 and 99.

The construction illustrated in Figure 9 offers the same advantages as the constructionof Figure 8 and for the same reasons. may be altered by moving the members 91 and 98 toward or away from one another. The natural frequency of the oscillating system may be altered by modifying the spring 99. l

As the armature assembly is supported very weakly in a vertical direction, 'it does not follow the vertical motion of the bell, and its mass need not be considered when determining the characteristics of a mechanical oscillating system.

From the foregoing it will be clear that novel signal generators have been provided which operate withv equal eiilciency in media under any pressure conditions and that such generators may bemade small enough to be lowered into bore holes of great depth and at the same time are capable of emitting pressure pulses o! suiilcient magnitude for the purpose desired. A

While several speciilc embodiments of the invention have been described in detail above, it will be understood that these embodiments are susceptible to considerable modification and that. therefore, they should be considered as illustrative of the invention and not as limiting the scope of the following claims.

I claim: f

1. An apparatus for emitting pressure pulses in a liquid comprising 'a first liquid-tight casing, a liquid-tight substantially rigid bell casing. means having capacity for elongation and contraction connecting said bell casing to said rst casing, an electromagnet in said bell casing, means in said first casing for supplying alternating current to said electromagnet, and armature means resiliently connected to said casing and,

adjacent to said electromagnet, whereby, upon energization of said electromagnet, said bell casing is reciprocated bodily with respect. to said rst casing. I

2. An apparatus for emitting pressure pulses in a liquid comprising a first liquid-tight casing, a substantially rigid, liquid-tight bell casing, means having capacity for elongation and contraction connecting said bell casing to said first casing, an electromagnet in said bell casing having a core, means including an oscillator in-said rst casing for supplying alternating current to said electromagnet, armature means disposed The couplingv l2 ture means. whereby upon energilation of said electromagnet, said bell casing is reciprocated bodily with respect to said first casing.

3. An apparatus for emitting pressure pulses in a liquid comprising a rst liquid-tight casing. a substantially rigid liquid-tight bell casing. means having capacity for elongation and contraction connecting said bell casing to said ilrst in a liquid, comprising a first liquid-tight casing.' a substantially rigid liquid-tight bell I casing,

means having capacity for elongation and contraction connecting said bell casing to said ilrst casing, an electromagnet in said bell casing. means in said first casing for supplying alternating current to said electromagnet, a pair of spaced apart armature arms on opposite sides of 'said electromagnet resiliently connected at one relatively close to said core, and a resilient memend to the end of said bell casing, whereby, upon energization of said electromagnet, said bell casing is reciprocated bodily with respect to said ilrst casing.

5. An apparatus for emitting pressure pulses in a liquid comprising a first liquid-tight casing, a substantially rigid liquid-tight bell casing, means having capacity for elongation and contraction connecting said bell casing to said ilrst casing, an electromagnet in said bell casing, means for resiliently supporting said electromagnet in vsaid bell casing to permit movement of said bell casing relatively to said electromagnet, means in said rst casing for supplying alternating current to said electromagnet, and armature means resiliently connected to said casing and disposed adjacent to said electromagnet, whereby, upon energization of said electromagnet. said bell casing is reciprocated bodily with respect to said ilrst casing.

6. An apparatus for emitting pressure pulses in a liquid comprising a support, a substantially rigid bell resiliently connected to said support. vibrator means in said bell for reciprocating said bell bodily with respect to said support in said liquid, and means responsive to vibration of said bell for maintaining the frequency of said bell at substantially its natural resonant frequency.

7. An apparatus for emitting pressure pulses in a liquid comprising a substantially rigid bell, electro-mechanical vibrator means in said bell for reciprocating said bell bodily in said liquid, a pickup means in said bell for supplying alternating current in response to vibration of said bell, and an amplier for supplying electrical energy to said vibrator, said pickup means being electrically connected to said amplifier to cause said bell to reciprocate at its resonant frequency.

8. An apparatus for emitting pressure pulses in a liquid comprising a ilrst liquid-tight casing, a substantially rigid liquid-tight bell casing. means resiliently connecting said casings for relative axial movement, an armature resiliently connected to said bell casing for vibration to provide an energy component axially of said bell, an electromagnet having a coil winding in said bell casing adjacent to said armature, an amplifier in i said ilrst casing adapted to be energized from an l 3 external source and connected to said electromagnet for energizing the same, and means supporting said coil winding resiliently in said bell casing.

9. An apparatus for emitting pressure pulses in a liquid comprising a rst liquid-tight casing, a substantially rigid liquid-tight bell, means resilientiy connecting said casing and said bell for relative axial movement. a pair of armatures secured to one end of and within said bell having end portions in spaced apart relationship, an electromagnet disposed between said portions and supported resiliently for movement axially of said bell, and an amplifier in said casing adapted to be energized from an external source for energizing said electromagnet to cause'said bell to be reciprocated bodily with respect to the casing.

10. An apparatus for emitting pressure pulses in a liquid comprising a rst liquid-tight casing, a substantially rigid liquid-tight bell, means resiliently connecting said. casing and said bell for relative axial movement. a pair of armatures secured to one end of and within said bell having end portions in spaced apart relationship, resilient means interposed between said armatures normally urgingl them apart, an electromagnet disposed between said end portions and supported resiliently for movementv axially of said bell, and an amplifier in said casing adapted to be energized from an external source for energizing said electromagnet. v

11. An apparatus for emitting signals in a liquid comprising a rst liquid-tight casing, a liq- .uid-tight, substantially rigid, hollow, bell casing,

means having a capacity for elongation and contraction connecting said bell casing to said rst casing, and vibrator means in said bell casing for reciprocating said bell casing bodily in said liquid with respect to said ilrst casing.

12. An apparatus for emitting pressure pulses in a liquid comprising a first liquid-tight casing. a liquid-tight, substantially rigid, hollow, bell casing, means having a capacity for elongation and contraction connecting `said bell casing to said rst casing, and vibrator means in said bell casing including an electromagnet, and means for energizing said electromagnet to reciprocate said bell casing bodily with respect to said first casing.

Y ANDR BLANCHARD.

REFERENCES crran The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 636,519 Gray et al. Nov. 7,1899 935,750 Gale Oct. 5, 1909 942,897 Garrett et al. Dec. 14, 1909 1,097,859 Hecht May 26, 1914 1,518,123 Lawther Dec. 2, 1924 1,526,414 Bois-Reymond Feb. 17, 1925 1,577,741 Bois-Reymond et a1. Mar. 23, 1926 1,604,693 Hecht et al. Oct. 26, 1926 1,872,946 Hecht Aug. 23, 1932 2,405,225 Mason Aug. 6, 1946 FOREIGN PATENTS Number Country Date 311,327 Great Britain June 5, 1930 704,486

France May 20, 1931 

